The invention relates to a multiplexed optical fiber transducer array for use in seismic exploration equipment. A seismic signal may be either an acoustic signal or an acceleration signal, depending on the application. The transducer array is formed as a string of transducer assemblies threaded on a fiber without splicing. Fiber optic sensors that respond to variations in a selected field quantity such as pressure, acceleration, etc. have shown increasing promise for the acquisition of vast quantities of data, particularly in seismic exploration applications. Known means for measuring such variations include interferometers which detect changes, in optical phase and spectrometers which detect spectral shifts. For example, certain fiber optic interferometric sensors respond to underwater perturbations such as acoustic wave fronts by varying the effective length of the fiber optic filament in response to the perturbation.
In such applications, optical fibers are made sensitive to these physical phenomena, such as acoustic waves and acceleration. An optical fiber exposed to such phenomena changes the medium through which a light or infrared beam passes that is guided by the fiber. Optical fibers have been considered for use as sensing elements and devices such as microphones, hydrophones, magnetometers, accelerometers, and electric current sensors.
A hydrophone array or acoustic sensor array may be formed as an integral, self-contained linear array of hydrophones on a single cable. Commonly, such an array is made up of electromechanical transducer elements, principally piezoelectric devices, which generate electrical signals in response to pressure variations. These conventional sensors typically are active devices that require many electrical wires or cables. The sensors have the disadvantage of being susceptible to electrical noise and signal cross talk.
Fiber optic Mach-Zehnder and Michelson interferometers respond to the phenomena being sent by producing phase differences in interfering light waves guided by the optical fibers. Detecting phase changes in the waves permits quantitative measurements to be made on the physical quantity being monitored.
A fiber optic Mach-Zehnder interferometer typically has a reference arm comprising of first length of optical fiber and a sensing arm comprised of a second length of optical fiber. The sensing arm is exposed to the physical parameter to be measured, such as an acoustic wave front, while the reference arm is isolated from changes in the parameter. When the Mach-Zehnder interferometer is used as an acoustic sensor, acoustic wave fronts change the optical length of the sensing arm as a function of the acoustic wave pressure amplitude. An optical coupler divides a light signal between the two arms. The signals are recombined after they have propagated through the reference and sensing arms, and the phase difference of the signals is monitored. Since the signals in the reference and sensing arms have a definite phase relation when they were introduced into the arms, changes in the phase difference are indicative of changes in the physical parameter to which the sensing arm was exposed.
A Michelson interferometer also has a sensing arm and a reference arm that propagates sensing and reference signals, respectively. However, in the Michelson interferometer, these arms terminate in mirrors that cause the sensing and reference signals to traverse their respective optical paths twice before being combined to produce an interference pattern.
While effective in changing the effective optical path length of the optical fiber in response to the phenomenon to be measured, these known structures are relatively complex and relatively delicate, and therefore subject to a variety of modes of damage in the harsh environment in which they are deployed.
As an alternative to the interferometer, other systems use frequency modulation technique for the detection of the parameter to be measured. In both the interferometer and frequency modulation techniques, the fiber may be modified with a Bragg grating. Optical fiber Bragg gratings commonly take the form of a periodic modulation of the refractive index along a short length of the fiber. The grating reflects light of one wavelength which satisfies the Bragg condition that the wavelength is twice the periodicity of the grating. The periodicity is altered if the temperature or strain environment of the fiber is changed and therefore these parameters may be measured by monitoring the reflected wavelength.
Thus, there remains a need for a sensor that is responsive to variations in pressure, in the case of a hydrophone, or to acceleration, in the case of a geophone, using variations in the stress on a fiber optic element that is both robust and easily manufactured. In the case of a hydrophone, if the distance between Bragg gratings varies with the stress, then interferometric techniques may be used. On the other hand, stressing the fiber at the point of the Bragg grating actually stretches the grating itself and causes a variation in the frequency to which the Bragg grating responds. In that case, frequency modulation techniques are used to detect and measure the physical parameter of interest.